mktemp

MKTEMP(1)MKTEMP(1)

NAME

mktemp - make temporary filename (unique)

SYNOPSIS

mktemp [-V] | [-dqtu] [-pdirectory] [template]

DESCRIPTION

The mktemp utility takes the given filename template and overwrites a
portion of it to create a unique filename. The template may be any
filename with some number of ‘Xs’ appended to it, for example
/tmp/tfile.XXXXXXXXXX. If no template is specified a default of
tmp.XXXXXXXXXX is used and the -t flag is implied (see below).
The trailing ‘Xs’ are replaced with a combination of the current pro-
cess number and random letters. The name chosen depends both on the
number of ‘Xs’ in the template and the number of collisions with
pre-existing files. The number of unique filenames mktemp can return
depends on the number of ‘Xs’ provided; ten ‘Xs’ will result in mktemp
testing roughly 26 ** 10 combinations.
If mktemp can successfully generate a unique filename, the file (or
directory) is created with file permissions such that it is only read-
able and writable by its owner (unless the -u flag is given) and the
filename is printed to standard output.
mktemp is provided to allow shell scripts to safely use temporary
files. Traditionally, many shell scripts take the name of the program
with the PID as a suffix and use that as a temporary filename. This
kind of naming scheme is predictable and the race condition it creates
is easy for an attacker to win. A safer, though still inferior
approach is to make a temporary directory using the same naming scheme.
While this does allow one to guarantee that a temporary file will not
be subverted, it still allows a simple denial of service attack. For
these reasons it is suggested that mktemp be used instead.
The options are as follows:
-V Print the version and exit.
-d Make a directory instead of a file.
-pdirectory
Use the specified directory as a prefix when generating the tem-
porary filename. The directory will be overridden by the user’s
TMPDIR environment variable if it is set. This option implies
the -t flag (see below).
-q Fail silently if an error occurs. This is useful if a script
does not want error output to go to standard error.
-t Generate a path rooted in a temporary directory. This directory
is chosen as follows:
· If the user’s TMPDIR environment variable is set, the
directory contained therein is used.
· Otherwise, if the -p flag was given the specified direc-
tory is used.
· If none of the above apply, /tmp is used.
In this mode, the template (if specified) should be a directory
component (as opposed to a full path) and thus should not contain any
forward slashes.
-u Operate in ‘‘unsafe’’ mode. The temp file will be unlinked
before mktemp exits. This is slightly better than mktemp(3) but
still introduces a race condition. Use of this option is not
encouraged.
The mktemp utility exits with a value of 0 on success or 1 on failure.

EXAMPLES

The following sh(1) fragment illustrates a simple use of mktemp where
the script should quit if it cannot get a safe temporary file.
TMPFILE=‘mktemp /tmp/example.XXXXXXXXXX‘ || exit 1
echo "program output" >> $TMPFILE
The same fragment with support for a user’s TMPDIR environment variable
can be written as follows.
TMPFILE=‘mktemp -t example.XXXXXXXXXX‘ || exit 1
echo "program output" >> $TMPFILE
This can be further simplified if we don’t care about the actual name
of the temporary file. In this case the -t flag is implied.
TMPFILE=‘mktemp‘ || exit 1
echo "program output" >> $TMPFILE
In some cases, it may be desirable to use a default temporary directory
other than /tmp. In this example the temporary file will be created in
/extra/tmp unless the user’s TMPDIR environment variable specifies oth-
erwise.
TMPFILE=‘mktemp -p /extra/tmp example.XXXXXXXXXX‘ || exit 1
echo "program output" >> $TMPFILE
In some cases, we want the script to catch the error. For instance, if
we attempt to create two temporary files and the second one fails we
need to remove the first before exiting.
TMP1=‘mktemp -t example.1.XXXXXXXXXX‘ || exit 1
TMP2=‘mktemp -t example.2.XXXXXXXXXX‘
if [ $? -ne 0 ]; then
rm -f $TMP1
exit 1
fi
Or perhaps you don’t want to exit if mktemp is unable to create the
file. In this case you can protect that part of the script thusly.
TMPFILE=‘mktemp -t example.XXXXXXXXXX‘ && {
# Safe to use $TMPFILE in this block
echo data > $TMPFILE
...
rm -f $TMPFILE
}

ENVIRONMENT

TMPDIR directory in which to place the temporary file when in -t mode

SEE ALSO

mkdtemp(3), mkstemp(3), mktemp(3)

HISTORY

The mktemp utility appeared in OpenBSD 2.1.
30 September 2001 MKTEMP(1)